1. Field of the Invention
This invention relates to an apparatus and process for control and maintenance of fluidized beds under non-steady state conditions. The desired control and maintenance of the fluidized bed of an ash agglomerating type is achieved by monitoring the ash content in the fluidized bed and providing a removal system dependent only upon the fluidized bed ash content for reduction of ash content of the fluidized bed in order to maintain desired control of fluidization. The controlled process of this invention provides stable, sinter-free fluidized bed operations before ash agglomeration is initiated and during upset conditions.
2. Description of the Prior Art
Fluidized bed processes are well known by the prior art in regard to their utility in converting solids to liquid and gaseous fuels. Ash agglomerating fluidized beds are taught by U.S. Pat. Nos. 3,935,825; 4,057,402; 4,229,289; and 4,369,045. In all of the processes taught by these patents, the removal of solids by downward passage through a Venturi withdrawal conduit in the lower portion of the grate supporting the fluidized bed is controlled by the velocity of upward flow of gas to the fluidized bed. The teachings of these disclosures are incorporated herein by reference. The above patents illustrate the importance of providing sufficient removal of agglomerated ash from fluidized bed coal gasifiers during steady state operations. Removal of ash provides additional working surface area and volume for solids conversion.
U.S. Pat. No. 4,099,933 teaches the necessity of maintaining char inventory in a coal gasifier fluidized bed to maintain desired balance between exothermic and endothermic reactions and teaches removal of ash and char through a straight conduit in the lowermost portion of the fluidized bed supporting grate. The U.S. Pat. No. 4,099,933, however, is not concerned with ash removal from an ash agglomerating fluidized bed.
It is also known to control the height of a fluidized bed by overflow pipes which control only the maximum volume of the bed and do not allow controlled withdrawal of solid particulates from the fluidized bed based upon the bed content of a specific particulate.
Removal of ash from a fluidized bed during start up or non-steady state conditions of an.ash agglomerating fluidized bed is of particular importance. It is during start up conditions or non-steady state upset conditions where control of the ash content of a fluidized bed becomes critical since the ash may sinter and cause defluidization of the bed. When this happens, the process must be shut down and the bed cleaned out. When ash levels reach a threshold concentration and where the fluidized bed conditions, such as bed temperature and oxidant gas concentration are appropriate, the ash particulates exhibit the propensity to sinter. Sintering of the ash disrupts the intended steady state balance of the system and significant amounts of sinter formation adversely affects the fluidization characteristics of the fluidized bed and may cause complete bed defluidization resulting in the shutdown of the entire process. Different coals, or other carbonaceous solids, result in differing ash contents and chemical characteristics which sinters under differing fluidized bed conditions, such as temperature. It is, therefore, desirable to provide a means to control the conditions within the fluidized bed to prevent sintering by a monitoring and rapid adjustment to provide a process adaptable to a wide variety of fluidized bed reactor vessels.